Permanent magnets are an important component in electric and electronic articles ranging from domestic electrical appliances to peripheral terminal devices of mainframe computers. Because of the trend towards miniaturization of high efficiency electric and electronic equipment, there is an increasing demand for strong permanent magnets that can be mass produced easily and economically. Accordingly, investigators continually are searching for relatively inexpensive and high performance permanent magnet materials. High performance permanent magnet materials generally exhibit relatively high magnetic properties, such as coercivity, remanence and maximum magnetic energy product.
A coercivity of about 1,000 Oe is the minimum required for a permanent magnet. Coercivity is a measure of the relative ease of magnetizing and demagnetizing a magnet material. A low coercivity is advantageous because a magnet material can be magnetized easily. However, a low coercivity is disadvantageous because the magnetized magnet material also can be demagnetized easily. Accordingly, a permanent magnet material having a high coercivity is preferred.
Remanence is the degree of magnetization retained by a magnet material after removing the applied field that magnetizes the magnet material. For example, a remanence of about 8,000 G (Gauss) is considered the state of the art for noninteractive, isotropic transition metal (TM)rare earth metal (RE)-boron (B) magnetic materials having an atomic proportional formula of RE.sub.2 TM.sub.14 B.sub.1. The maximum magnetic energy product (BH.sub.max) is another measurement of magnetic strength. The highest theoretical BH.sub.max for perfectly aligned (anisotropic) magnetic materials is 64 MGOe (megaGaussOersteds). For unaligned (isotropic) materials, the highest theoretical BH.sub.max is 16 MGOe. For unaligned materials, the highest practical value for BH.sub.max is about 14.2 MGOe, and a typical BH.sub.max value is about 12 MGOe.
Magnetic materials having high magnetic properties are called "hard"; whereas magnetic materials having low magnetic properties are called "soft". Metallic alloys used as magnetic materials can be either hard or soft. Presently, a wide variety of hard magnetic materials (also termed permanent magnetic materials) are known and used in permanent magnets. However, every known permanent magnetic material which possesses high magnetic properties also has a high cost. A permanent magnet material having a reasonable cost is suitable for merely some applications, but is unsuitable for other applications.
The highest performance permanent magnets are manufactured from intermetallic compounds or alloys comprising (1) a rare earth metal, and (2) a transition metal, such as samarium-cobalt alloys, like SmCo.sub.5, or similar alloys. These alloys possess sufficiently high magnetic properties for use in almost every application. However, because such alloys have the disadvantage of including a high percentage of very expensive metals, like cobalt, they are unsuitable for mass producing low cost magnets. In addition, alloys like samarium-cobalt alloys require a very complicated processing procedure to achieve maximum performance. A third disadvantage of these alloys is that the alloys exhibit a high coercivity over only a limited compositional range, thereby inherently limiting the ability of an investigator to alter other magnetic properties of the alloys, such as saturation magnetization, by changing the proportions of ingredients.
A magnet material that does not include a rare earth metal typically exhibits a substantially lower coercivity than samarium-cobalt and related alloys. The various forms of ALNICO, for example, exhibit a coercivity of about 600 to about 1400 0e. A coercivity in this range is too low for many applications because the magnet material is demagnetized too easily. ALNICO alloys also have the disadvantage of including a relatively large amount of cobalt, which is expensive.
Ferrites, which contain iron oxides, also are used extensively as permanent magnet materials. Various classes of ferrite are available very cheaply, but ferrites typically have a low remanence and possess a moderate coercivity. The main advantage of ferrites therefore is a very low cost, which makes the mass production of ferrite-containing permanent magnets practical.
Because of the disadvantages exhibited by prior permanent magnet materials, investigators sought a permanent magnet material that outperforms the ferrites, and that is less expensive than the samarium-cobalt alloys. One such class of permanent magnet materials comprises one or more transition metals, one or more rare earth metals, and boron. Prior transition metal-rare earth metal-boron (TM--RE--B) alloys predominantly included a "2-14-1" phase, i.e., RE.sub.2 TM.sub.14 B.sub.1, wherein the subscripts denote the atomic proportions of the components (alternatively expressed as TM.sub.82 RE.sub.12 B.sub.6, wherein the subscripts denote the atomic percentages (atomic %) of the components). The RE.sub.2 TM.sub.14 B.sub.1 alloy has a uniaxial crystal structure, i.e., a single easy axis of magnetization. Conventionally, investigators have maintained that a permanent magnet material must be crystallographically uniaxial in order to exhibit permanent magnetic properties.
To date, non-uniaxial TM--RE--B alloys have not been used in permanent magnets because the non-uniaxial TM--RE--B alloys exhibited a coercivity of less than 1,000 Oe. Investigators have been unable to provide a TM--RE--B alloy of sufficiently high coercivity (i.e., at least 1,000 Oe) when the magnet material includes too large a fraction of non-uniaxial material. Therefore, prior TM--RE--B alloys with substantial non-uniaxial material (e.g., more than 10 weight %) possessed no intrinsic permanent magnetic properties. Surprisingly and unexpectedly, a permanent magnet material of the present invention, including at least 10%, and typically at least 40%, by weight non-uniaxial material, exhibits sufficiently high magnetic properties for use in a permanent magnet.
Therefore, prior permanent magnets could be manufactured from 1) the highly effective but very expensive samarium-cobalt alloys, which limit mass production of the permanent magnets because of raw material cost, or 2) the ferrites, which are economical but do not exhibit sufficient magnetic properties for many permanent magnet applications. In addition, between these extremes there were permanent magnets manufactured from the TM--RE--B permanent magnetic materials, and most notably the TM--RE--B materials having predominantly the RE.sub.2 TM.sub.14 B.sub.1 phase.
TM--RE--B alloys are disclosed in numerous patents. For example, Koon U.S. Pat. Nos. 4,374,665, 4,402,770, 4,409,043, and 4,533,408 disclose a magnetic alloy comprising (1) iron (a transition metal), (2) lanthanum and a lanthanide (rare earth metals) and (3) boron.
Croat U.S. Pat. Nos. 4,802,931 and 4,851,058 disclose predominantly single phase TM--RE--B alloys with the "2-14-1" phase (i.e., RE.sub.2 TM.sub.14 B.sub.1) as the single phase. Croat U.S. Pat. No. 4,496,395 discloses a two component magnet material including a rare earth metal and a transition metal. Similarly, Brewer et al. U.S. Pat. No. 4,881,985 discloses a uniaxial TM--RE--B alloy comprising predominantly the RE.sub.2 TM.sub.14 B.sub.1 phase.
Other patents disclosing TM--RE--B magnets include:
Mizoguchi et al. U.S. Pat. No. 5,071,493, disclosing a two-phase permanent magnet comprising a sintered alloy having a magnetic RE.sub.2 TM.sub.14 B.sub.1 phase and a non-magnetic phase that is rich in the rare earth metals; PA1 De Mooij et al. U.S. Pat. No. 4,935,074, disclosing a magnetic material comprising a transition metal, a rare earth metal and boron, wherein the rare earth metal is present in an amount of 4.8 atomic % or less, and the boron is present in an amount of 16 to 26 atomic %; PA1 Sagawa et al. U.S. Pat. No. 4,770,723, disclosing a magnetically anisotropic TM--RE--B magnetic material having a tetragonal crystal structure; PA1 Keem et al. U.S. Pat. Nos. 4,867,785 and 5,116,434, disclosing methods of manufacturing magnetic materials; PA1 Ovshinsky et al. U.S. Pat. Nos. 4,715,891 and 4,753,675, disclosing methods of preparing a magnetic material, such as a RE.sub.2 TM.sub.14 B.sub.1 phase TM--RE--B alloy, either including or excluding modifiers, like silicon or aluminum; and PA1 European Patent Application 0 195 219, disclosing TM--RE--B alloys further including a silicon or aluminum modifier.
A permanent magnet composed of TM--RE--B contains particles of the alloy and comprise either 1) a magnetically aligned permanent magnet material, wherein the particles are referred to as aligned or anisotropic, or 2) a magnetically unaligned permanent magnet material, wherein the particles are referred to as unaligned or isotropic. A permanent magnet comprising aligned particles generally has a substantially greater magnetic strength than a magnet comprising unaligned particles. In a permanent magnet comprising aligned particles, the magnetic strength is directional. A substantial magnetic strength exists in the direction of alignment, whereas very little magnetic strength exists in directions transverse to the direction of alignment. In contrast, in a permanent magnet comprising unaligned particles, the magnetic strength is substantial in all directions, but the magnetic strength in any direction is generally considerably less than the magnetic strength in the direction of alignment in a magnet composed of aligned particles.
Accordingly, it would be desirable to provide an inexpensive, isotropic permanent magnet material that exhibits the magnetic properties of an expensive, aligned permanent magnet material. It also would be desirable to provide a permanent magnet material that is easy to magnetize (i.e., has a low coercivity, like ferrite), but retains a high degree of magnetization (i.e., has a high remanence, like a predominantly aligned TM--RE--B magnet material). A permanent magnet material having the combination of low coercivity and high remanence permits a permanent magnet composed of the material to be magnetized after the permanent magnet is positioned within a device.